Mechanism of a transcriptional cross talk between transforming growth factor-β-regulated Smad3 and Smad4 proteins and orphan nuclear receptor hepatocyte nuclear factor-4

We have shown previously that the transforming growth factor-P (TGFbeta)-regulated Sma-Mad (Smad) protein 3 and Smad4 proteins transactivate the apolipoprotein C-M promoter in hepatic cells via a hormone response element that binds the nuclear receptor hepatocyte nuclear factor 4 (HNF-4). In the present study, we show that Smad3 and Smad4 but not Smad2 physically interact with HNF4 via their Mad homology I domains both in vitro and in vivo. The synergistic transactivation of target promoters by Smads and HNF4 was shown to depend on the specific promoter context and did not require an intact beta-hairpin/ DNA binding domain of the Smads. Using glutathione S-transferase interaction assays, we established that two regions of HNF4, the N-terminal. activation function 1 (AF-1) domain (aa 1-24) and the C-terminal F domain (aa 388-455) can mediate physical Smad3/HNF-4 interactions in vitro. In vivo, Smad3 and Smad4 proteins enhanced the transactivation function of various GAL4-HNF-4 fusion proteins via the AF-1 and the adjacent DNA binding domain, whereas a single tyrosine to alanine substitution in AF-1 abolished coactivation by Smads. The findings suggest that the transcriptional cross talk between the TGFbeta-regulated Smads and HNF4 is mediated by specific functional domains in the two types of transcription factors. Furthermore, the specificity of this interaction for certain target promoters may play an important role in various hepatocyte functions, which are regulated by TGFbeta and the Smads.